127 research outputs found
Information of Structures in Galaxy Distribution
We introduce an information-theoretic measure, the Renyi information, to
describe the galaxy distribution in space. We discuss properties of the
information measure, and demonstrate its relationship with the probability
distribution function and multifractal descriptions. Using the First Look
Survey galaxy samples observed by the Infrared Array Camera onboard Spitzer
Space Telescope, we present measurements of the Renyi information, as well as
the counts-in-cells distribution and multifractal properties of galaxies in
mid-infrared wavelengths. Guided by multiplicative cascade simulation based on
a binomial model, we verify our measurements, and discuss the spatial selection
effects on measuring information of the spatial structures. We derive structure
scan functions at scales where selection effects are small for the Spitzer
samples. We discuss the results, and the potential of applying the Renyi
information to measuring other spatial structures.Comment: 25 pages, 8 figures, submitted to ApJ; To appear in The Astrophysical
Journal 2006, 644, 678 (June 20th
Non-linear Stochastic Galaxy Biasing in Cosmological Simulations
We study the biasing relation between dark-matter halos or galaxies and the
underlying mass distribution, using cosmological -body simulations in which
galaxies are modelled via semi-analytic recipes. The nonlinear, stochastic
biasing is quantified in terms of the mean biasing function and the scatter
about it as a function of time, scale and object properties. The biasing of
galaxies and halos shows a general similarity and a characteristic shape, with
no galaxies in deep voids and a steep slope in moderately underdense regions.
At \sim 8\hmpc, the nonlinearity is typically \lsim 10 percent and the
stochasticity is a few tens of percent, corresponding to percent
variations in the cosmological parameter . Biasing
depends weakly on halo mass, galaxy luminosity, and scale. The time evolution
is rapid, with the mean biasing larger by a factor of a few at
compared to , and with a minimum for the nonlinearity and stochasticity at
an intermediate redshift. Biasing today is a weak function of the cosmological
model, reflecting the weak dependence on the power-spectrum shape, but the time
evolution is more cosmology-dependent, relecting the effect of the growth rate.
We provide predictions for the relative biasing of galaxies of different type
and color, to be compared with upcoming large redshift surveys. Analytic models
in which the number of objects is conserved underestimate the evolution of
biasing, while models that explicitly account for merging provide a good
description of the biasing of halos and its evolution, suggesting that merging
is a crucial element in the evolution of biasing.Comment: 27 pages, 21 figures, submitted to MNRA
Galaxy formation in the Planck cosmology - II. Star-formation histories and post-processing magnitude reconstruction
We adapt the L-Galaxies semi-analytic model to follow the star-formation
histories (SFH) of galaxies -- by which we mean a record of the formation time
and metallicities of the stars that are present in each galaxy at a given time.
We use these to construct stellar spectra in post-processing, which offers
large efficiency savings and allows user-defined spectral bands and dust models
to be applied to data stored in the Millennium data repository.
We contrast model SFHs from the Millennium Simulation with observed ones from
the VESPA algorithm as applied to the SDSS-7 catalogue. The overall agreement
is good, with both simulated and SDSS galaxies showing a steeper SFH with
increased stellar mass. The SFHs of blue and red galaxies, however, show poor
agreement between data and simulations, which may indicate that the termination
of star formation is too abrupt in the models.
The mean star-formation rate (SFR) of model galaxies is well-defined and is
accurately modelled by a double power law at all redshifts: SFR proportional to
, where Gyr, is the age of the
stars and is the loopback time to the onset of galaxy formation; above a
redshift of unity, this is well approximated by a gamma function: SFR
proportional to , where Gyr. Individual
galaxies, however, show a wide dispersion about this mean. When split by mass,
the SFR peaks earlier for high-mass galaxies than for lower-mass ones, and we
interpret this downsizing as a mass-dependence in the evolution of the quenched
fraction: the SFHs of star-forming galaxies show only a weak mass dependence.Comment: Accepted version of the paper, to appear in MNRAS. Compared to the
original version, contains more detail on the post-processing of magnitudes,
including a table of rms magnitude errors. SFHs available on Millennium
database http://gavo.mpa-garching.mpg.de/MyMillennium
Orbital Parameters of Merging Dark Matter Halos
In order to specify cosmologically motivated initial conditions for major
galaxy mergers (mass ratios 4:1) that are supposed to explain the
formation of elliptical galaxies we study the orbital parameters of major
mergers of cold dark matter halos using a high-resolution cosmological
simulation. Almost half of all encounters are nearly parabolic with
eccentricities and no correlations between the halo spin planes
or the orbital planes. The pericentric argument shows no correlation
with the other orbital parameters and is distributed randomly. In addition we
find that 50 % of typical pericenter distances are larger than half the halo's
virial radii which is much larger than typically assumed in numerical
simulations of galaxy mergers. In contrast to the usual assumption made in
semi-analytic models of galaxy formation the circularities of major mergers are
found to be not randomly distributed but to peak around a value of . Additionally all results are independent of the minimum
progenitor mass and major merger definitions (i.e. mass ratios 4:1; 3:1;
2:1).Comment: 11 pages, 20 figures, replaced by version accepted to A&A, figure 1
low re
Evaluating Semi-Analytic Halo Merging Histories
We evaluate the accuracy of semi-analytic merger-trees by comparing them with
the merging histories of dark-matter halos in N-body simulations, focusing on
the joint distribution of the number of progenitors and their masses. We first
confirm that the halo mass function as predicted directly by the
Press-Schechter (PS) model deviates from the simulations by up to 50% depending
on the mass scale and redshift, while the means of the projected distributions
of progenitor number and mass for a halo of a given mass are more accurately
predicted by the Extended PS model. We then use the full merger trees to study
the joint distribution as a function of redshift and parent-halo mass. We find
that while the deviation of the mean quantities due to the inaccuracy of the
Extended PS model partly propagates into the higher moments of the
distribution, the merger-tree procedure does not introduce a significant
additional source of error. In particular, certain properties of the merging
history such as the mass ratio of the progenitors and the total accretion rate
are reproduced quite accurately for galaxy sized halos (\sim 10^{12}\msun),
and less so for larger masses. We conclude that although there could be deviations in the absolute numbers and masses of progenitors and in the
higher order moment of these distributions, the relative properties of
progenitors for a given halo are reproduced fairly well by the merger trees.
They can thus provide a useful framework for modelling galaxy formation once
the above-mentioned limitations are taken into account.Comment: 10 pages including 9 figures, submitted to MNRA
Origin, diversification, and classification of the Australasian genus dracophyllum (richeeae, ericaceae)
The genus Dracophyllum Labill. (Ericaceae) has a fragmented distribution in Australasia, but reaches the greatest level of species richness and morphological diversity in New Zealand. We investigated evolutionary processes that contribute to this disparity in species richness by comparing DNA sequences from members of Dracophyllum, its close relatives Richea Labill. and Sphenotoma R. Br. ex Sweet (together constituting tribe Richeeae Crayn & Quinn), along with more distant relatives in the Ericaceae. We created complementary data sets for the chloroplast-encoded genes matK and rbcL. Parsimony, Bayesian, and maximum likelihood analyses were conducted to assess the robustness of our phylogenetic inferences. The results were largely congruent and, when analyzed in combination, provided greater resolution. In our analyses, tribe Richeeae formed a monophyletic group that diverged during the Eocene (at least 33.3 million years ago [Ma]) with a crown radiation during the Early Miocene (at least 16.5 Ma) that resulted in two disjunct lineages. This date corresponds roughly to the onset of aridification in central Australia. The southern Western Australian genus Sphenotoma formed an isolated evolutionary lineage, while Dracophyllum and Richea together formed a second lineage restricted to eastern Australia, Lord Howe Island, New Caledonia, and New Zealand. The relationships of the Tasmanian endemic, D. milliganii Hook. f., remain an enigma. It was ambiguously placed as sister to Sphenotoma or to the Dracophyllum Richea elude. We recovered two distinct lineages, traditionally recognized as Richea sect. Cystanthe (R. Br.) Benth. and Richea sect. Dracophylloides Benth., which were nested within Dracophyllum. The Lord Howe Island endemic, D. fitzgeraldii F. Muell., emerged as sister to an eastern Australian clade of Dracophyllum. Our evidence suggests that the New Caledonian and New Zealand species of Dracophyllum dispersed from Australia; we document two independent episodes of long-distance dispersal in the Late Miocene to Early Pliocene. Low levels of sequence divergence suggest a rapid and recent species radiation in these two island archipelagos largely within the last three to six million years. This radiation accompanied Pliocene uplift of the New Zealand Southern Alps and episodes of glaciation during the Pleistocene. Because Dracophyllum is paraphyletic and Richea is polyphyletic, the taxonomic circumscription of these genera requires revision
A PCA-based automated finder for galaxy-scale strong lenses
We present an algorithm using Principal Component Analysis (PCA) to subtract
galaxies from imaging data, and also two algorithms to find strong,
galaxy-scale gravitational lenses in the resulting residual image. The combined
method is optimized to find full or partial Einstein rings. Starting from a
pre-selection of potential massive galaxies, we first perform a PCA to build a
set of basis vectors. The galaxy images are reconstructed using the PCA basis
and subtracted from the data. We then filter the residual image with two
different methods. The first uses a curvelet (curved wavelets) filter of the
residual images to enhance any curved/ring feature. The resulting image is
transformed in polar coordinates, centered on the lens galaxy center. In these
coordinates, a ring is turned into a line, allowing us to detect very faint
rings by taking advantage of the integrated signal-to-noise in the ring (a line
in polar coordinates). The second way of analysing the PCA-subtracted images
identifies structures in the residual images and assesses whether they are
lensed images according to their orientation, multiplicity and elongation. We
apply the two methods to a sample of simulated Einstein rings, as they would be
observed with the ESA Euclid satellite in the VIS band. The polar coordinates
transform allows us to reach a completeness of 90% and a purity of 86%, as soon
as the signal-to-noise integrated in the ring is higher than 30, and almost
independent of the size of the Einstein ring. Finally, we show with real data
that our PCA-based galaxy subtraction scheme performs better than traditional
subtraction based on model fitting to the data. Our algorithm can be developed
and improved further using machine learning and dictionary learning methods,
which would extend the capabilities of the method to more complex and diverse
galaxy shapes
Interpretation of the transpulmonary thermodilution curve in the presence of a left-to-right shunt
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A Candidate Brightest Proto-Cluster Galaxy at z = 3.03
We report the discovery of a very bright (m_R = 22.2) Lyman break galaxy at z
= 3.03 that appears to be a massive system in a late stage of merging. Deep
imaging reveals multiple peaks in the brightness profile with angular
separations of ~0.''8 (~25 h^-1 kpc comoving). In addition, high
signal-to-noise ratio rest-frame UV spectroscopy shows evidence for ~5
components based on stellar photospheric and ISM absorption lines with a
velocity dispersion of sigma ~460 km s^-1 for the three strongest components.
Both the dynamics and high luminosity, as well as our analysis of a LCDM
numerical simulation, suggest a very massive system with halo mass M ~ 10^13
M_solar. The simulation finds that all halos at z = 3 of this mass contain
sub-halos in agreement with the properties of these observed components and
that such systems typically evolve into M ~ 10^14 M_solar halos in groups and
clusters by z = 0. This discovery provides a rare opportunity to study the
properties and individual components of z ~ 3 systems that are likely to be the
progenitors to brightest cluster galaxies.Comment: 14 pages, 3 figures, submitted to ApJ Letter
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